The Eruptive Tempo of Deccan Volcanism in Relation to The
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RESEARCH 40 39 MASS EXTINCTION We measured 19 high-precision Ar/ Ar ages for the DT that complement the previous geo- chronology to create a higher-resolution tempo- ral framework for DT volcanism. We collected The eruptive tempo of Deccan samples for geochronological analysis from the Western Ghats region of the DT, the most rele- volcanism in relation to the vant region for understanding DT-induced climate change, as the record of the most voluminous Cretaceous-Paleogene boundary eruptive phase of the DT occurs here (Fig. 1). The total lava stratigraphy is called the Deccan Group, 1,2 1,3 4 5 which is divided into formations within the larger Courtney J. Sprain *, Paul R. Renne , Loÿc Vanderkluysen , Kanchan Pande , Stephen Self1, Tushar Mittal1 Kalsubai, Lonavala, and Wai subgroups (in ascend- ing order) (Fig. 1). These formational and subgroup boundaries arise from geochemical and volcano- Late Cretaceous records of environmental change suggest that Deccan Traps logical properties (23). Each formation comprises (DT) volcanism contributed to the Cretaceous-Paleogene boundary (KPB) multiple eruptive units. In total, we sampled each ecosystem crisis. However, testing this hypothesis requires identification of subgroup and all but two formations within these the KPB in the DT. We constrain the location of the KPB with high-precision subgroups, including the stratigraphically high- argon-40/argon-39 data to be coincident with changes in the magmatic plumbing est and lowest dated samples from the Western system. We also found that the DT did not erupt in three discrete large pulses and Ghats. Our samples came from multiple sections. that >90% of DT volume erupted in <1 million years, with ~75% emplaced post-KPB. We focused sampling near the Lonavala–Wai Downloaded from Late Cretaceous records of climate change coincide temporally with the eruption Subgroup transition, the hypothesized location of the smallest DT phases, suggesting that either the release of climate-modifying oftheKPBbecauseofchangesinlavaflowmor- gases is not directly related to eruptive volume or DT volcanism was not the source phology, flow-field volumes, and geochemistry of Late Cretaceous climate change. ascribed to effects of the Chicxulub impact (7, 17). We collected samples to fill in the sampling gaps he mass extinction at the Cretaceous- and marine faunas (11–15). Furthermore, a tem- from Renne et al.(7)andSchoeneet al.(8), which http://science.sciencemag.org/ Paleogene boundary (KPB) fundamen- poral correlation between other flood basalt allows testing and improvement of their geo- tally reshaped Earth’s biosphere, ending eruptions and major ecological crises in Earth’s chronological models. the >150-million-year Age of the Dinosaurs history suggests the potential for the DT to have The 40Ar/39Ar method dates the eruption of T 16 and paving the way for the rise and dom- caused the mass extinction alone ( ). Additional lavas without the need for assumptions about inance of mammalian fauna. Understanding this hypotheses suggest a connection between the pre-eruptive residence time or provenance, as event is important for several reasons, including two mechanisms. Richards et al.(17)hypothesized are required for U-Pb zircon dates. We analyzed its implications for mammalian evolution and that major transitions in lava flow morphology, samples in detailed (11- to 21-step) step-heating the effects of abrupt climate change. Hypotheses flow-field volume, and feeder-dike orientation experiments with multigrain aliquots of plagi- regarding the cause of the mass extinction center observed within the DT stratigraphy were a re- oclase separates (fig. S1). To achieve high pre- around two potential triggers, invoking one or sult of a reorganization of the magmatic plumb- cision, we analyzed three to eight aliquots per both: voluminous flood basalt volcanism (total- ing system triggered by seismic energy from the sample (fig. S1), densely bracketed by standards on February 24, 2019 ing >106 km3 of magma) from the Deccan Traps Chicxulub impact (7, 17), overall enhancing vol- during irradiation in order to precisely deter- (DT) (in modern-day India) and the large bolide canism in the DT around the time of the KPB. mine the neutron fluence (19). We report here impact recorded by the Chicxulub crater. The Central to the hypothesis that the DT played the weighted mean plateau ages for each of our impact hypothesis is supported by the Chicxulub a contributing role in the mass extinction is samples (table S1) [errors are reported at an SEM crater (which coincides in age with the main ex- the assumption that large amounts of climate- of 1-s,withX versus Y indicating analytic versus tinction event) (1) and by a global KPB impact modifying gases (CO2,CH4,andSO2) were re- systematic uncertainty, respectively, per (24)]. ejecta layer (consisting of an iridium anomaly, leased by the Deccan before the KPB. Earlier When we combined our data with previously spherules, shocked minerals, and Ni spinels) geochronological studies (10, 18, 19) suggested published high-precision dates (7), we found (2–4), in addition to evidence of abrupt extinc- that the DT erupted in three phases, with ~80% that the DT lavas erupted quasi-continuously for tion recorded by marine microfossils and terres- of the extrusive volume erupting in phase 2, a 991,000 years (see Fig. 2), from ~66.413 Ma ago trial pollen and spores [e.g., (5, 6)]. Support for short pulse starting ~400,000 years before the [the date for Jawhar Formation (Fm.) sample the Deccan hypothesis includes geochronologic KPB and ending at the boundary. Phase 2 is often KAS15-3] to ~65.422 Ma ago (the date for upper evidence that much of the DT erupted around the cited as the source of Late Cretaceous environ- Mahabaleshwar Fm. sample PAN15-3). This time KPB within a time span of ~1 million years (Ma) mental change (12, 14, 20–22). With the proposed interval represents a total estimated volume of (dominantly within chron C29r) (7–10), and DT location of the KPB as suggested from the tran- ~560,000 km3 [on the basis of volume estimates volcanism roughly coincides with Late Cretaceous sitions described above, attributed to the effects presented in (17), where estimates are weighted records of climate change, in addition to records of the Chicxulub impact (7, 17), it has also been by areal extent], including ~93% of the total of ecological stress observed in some terrestrial hypothesized that >75% of the volume of the DT estimated volume of the DT. By using our com- erupted post-KPB. Current high-precision geo- posite stratigraphic section, we determined from chronology [U-Pb (8)and40Ar/39Ar (7)] cannot our data that ~85% of the Kalsubai Subgroup 1Department of Earth and Planetary Science, University of adequately test among these hypotheses because erupted in a period of 242,000 ± 101,000 years, California, Berkeley, 307 McCone Hall, Berkeley, CA 94720- of sampling gaps that fail to pinpoint the KPB that ~95% of the Lonavala Subgroup erupted 2 4767, USA. Geomagnetism Laboratory, Department of Earth, within the Deccan lava stratigraphy. To better in 46,000 ± 129,000 years, and that ~95% of the Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 7ZE, UK. 3Berkeley Geochronology Center, understand the role of DT volcanism in end- Wai Subgroup erupted in a period of 690,000 ± 2455 Ridge Road, Berkeley, CA 94709, USA. 4Department of Cretaceous environmental change and mass ex- 185,000 years. Biodiversity, Earth and Environmental Science, Drexel tinction,herewereporthigh-resolution40Ar/39Ar From our age estimates for the Jawhar Fm., we University, 3245 Chestnut Street, PISB 123, Philadelphia, PA 5 plagioclase ages from the DT that locate the KPB found no evidence for older eruptions [the basis 19104, USA. Department of Earth Sciences, Indian Institute “ ” of Technology Bombay, Powai, Mumbai 400 076, India. and better refine the timing and tempo of erup- for phase 1 and the proposed Latifwadi Fm. *Corresponding author. Email: [email protected] tive fluxes. (10, 18, 19)]. Additionally, with our new data, we Sprain et al., Science 363, 866–870 (2019) 22 February 2019 1of5 RESEARCH | REPORT Downloaded from http://science.sciencemag.org/ Fig. 1. Stratigraphy and location map. (A) Dated horizons within was adapted from (9, 10). Stratigraphy for the ABO (Amboli Ghat) stratigraphic sections. Ages reported in millions of years are shown with section was adapted from (32). Stratigraphy for the MSJ (Malshej Ghat) 1-s uncertainties (SEM). Ages shown in blue are new ages reported section was acquired in (7). Stratigraphic information for the KJA in this study, and ages in black were reported in (7). Sample names for (Katraj Ghat) and VER (Varandha Ghat) sections was acquired in this dated horizons are given in parentheses. Dates for MG7 and BOR14-1 study. asl, above sea level. (B) Location map of samples collected in this are the weighted mean ages combined with results presented in (7). study. (C) Sampled traverses within the Western Ghats region. Orange Stratigraphy for sections KAS (Shahapur-Igatpuri), MAT (Matheran- dots indicate samples presented in this study, and red dots indicate Neral), BOR (Khopoli-Khandala), and AMB (Mahabaleshwar-Poladpur) samples presented in (7). on February 24, 2019 have further confirmed that the KPB is not lo- formations at the top of the Lonavala Subgroup Poladpur boundary are due to changes in the cated at the contact between the Mahabaleshwar (17), we employed the Bayesian age model “Bacon” magmatic system caused by the seismic energy and Ambenali formations but is lower in the (26). Because of uncertainties in volume estimates from the Chicxulub impact. stratigraphy and that no obvious, long hiatus and possibly diachronous formation contacts By using the above-described placement for in eruption occurred near the KPB. By con- between sections, the most secure application the KPB, we determined a mean magma extru- trast, we find an enhanced period of eruption of such models is to data from a continuous sion rate of 0.4 ± 0.1 km3/year, representing around the time of the KPB, demonstrated by stratigraphic section.